A typical apparatus for the manufacture of ceramic ware comprises a jigger having a rotating means on which a bottom mold is disposed and an arm thereabove having a top mold attached thereto. Ceramic ware articles are formed between the bottom mold and the top mold.
The jigger machines are of two types, those on which the rotating means is halted after each article is formed, thereby allowing the bottom mold to be installed thereon, and those on which the rotating means rotates continuously without halting, so that the bottom mold must be removed from and installed on the rotating means while it is rotating. For both types of jigger machines, there is a problem with the longevity of the bottom mold, which is better understood with reference to the process for manufacturing cermaic ware. For clarification, as used herein, the term "bottom mold" refers to the mold rotated on the jigger rotating means, and not to whether the rotating mold forms the top or bottom surface of the ceramic ware article. Similarly, the term "top mold" refers to the mold disposed on the arm of the jigger machine, and not to which surface of a ceramic article is formed therewith.
In the first step of the manufacture of ceramic plates and the like, potter's clay is placed on the bottom mold, either while the mold is rotating or while it is stationary, depending upon the type of jigger. The top mold is placed over the potters clay, thereby pressing the clay between the top and bottom molds. The bottom mold is then rotated at a predetermined speed, or it continues to rotate at such speed, and the clay is formed into the desired shape.
The thickness of the ceramic article is determined by the distance between the bottom mold and top mold after the spinning step on the jigger is completed. Typically, the arm bearing said top mold is lowered towards the bottom mold to a predetermined position which is independent of the height of the bottom mold. Therefore, if the vertical position of the bottom mold changes, the thickness of the ceramic articles will correspondingly change. Such changes in the vertical position of the bottom mold can be explained as follows.
The bottom mold is generally made from a plaster material which has specific desirable physical characteristics that allow the mold to interact with the clay in a particular way. Thus, it is necessary for the mold surface contacting the clay to be made of a plaster material. Conversely, the rotating means, which engages the layer surface of said bottom mold, is generally formed of metal. Thus, when said bottom mold is placed on the rotating means while it is rotating, or is removed therefrom, wear occurs at the junction at which said bottom mold engages said rotating means. Similarly, for the type of jigger which halts its rotation so that the mold can be installed or removed, therefrom, when the rotating means is rapidly accelerated to a predetermined speed, until said bottom mold achieves the same velocity as said rotating means, wear occurs at the junction at which said bottom mold engages said rotating means. This wear results in the cutting of a groove in the mold at the surface which engages the rotating means. As the depth of the groove increases, the height of the bottom mold decreases. Correspondingly, since the vertical position of the top mold is preset, the size of the gap between the top and bottom molds increases, and consequently, the thickness of the ceramic ware article formed therebetween increases.
Therefore, it becomes necessary to replace said bottom mold with relative frequency, in order to keep the thickness of ceramic ware formed by this process within predefined tolerances. Such frequent mold replacement has the obvious consequence of increasing the manufacturing costs of ceramic ware. The present invention solves this problem of frequent mold replacement.
Moreover, it is also important in producing uniformly shaped ceramic ware, that the mold rotate substantially circularly. If the mold rotates off-center, or is wobbling about its axis of rotation, the clay thereon will be lop-sided, and an irregularly shaped article will be formed. This wobble may be caused by excessive wear on the positioning stem of the bottom mold which seats in the rotating means due to friction between said mold and said rotating means. The present invention also solves the aforementioned problem.